Consider the Parasite

When you hear the word parasite, you might picture a slimy leech, a creepy-crawly tick, or a bloodsucking bodily invader. Many parasites seem gross, and some are very dangerous for humans. But did you know that they are also incredibly important? They help keep the world’s ecosystems balanced.

Parasites come in a dazzling array of forms. They can be viruses, bacteria, fungi, plants, or animals. Parasites range in size from microscopic germs to 130-foot-long worms that live inside whales. Despite these differences, parasites all share one thing in common: They live in or on another living thing, called a host. Parasites make their host sick, weak, or really uncomfortable. 

They do this in surprising ways. The trypanorhynch tapeworm looks like a long, smooth mushroom. It also has four tentacles that shoot out of its face! This feature isn’t just for show; it’s for survival. The tapeworm lives in the dark, slippery gut of a shark, where it must avoid getting pooped out. That’s where the face-exploding trick comes in. Each launched tentacle is covered in thousands of backward-facing spines. Those spines hook on to the lining of the shark’s intestine. Attaching itself like this is while happily stealing nutrients from the shark.  

Amazingly adapted and wondrously weird parasites are all around us. They inhabit our pets and our favorite wildlife. They are in the food we eat, in the dirt we walk on, and in the rivers, lakes, and oceans we swim in. But contrary to popular belief, the vast majority of parasites have no direct contact with humans. “They’re just living their lives out in nature,” said Chelsea Wood, a parasite ecologist at the University of Washington and author of Power to the Parasites!—a book about her favorite subject, written just for kids. 

Lots of parasites have life cycles that rely on them infecting multiple hosts. A tapeworm, for example, starts out as an egg that a previously infected dog or cat poops out into the environment. The tapeworm egg will hatch into a larva only if it’s lucky enough to be eaten by a wormlike baby flea (itself a parasite with its own life cycle). The tapeworm larva hangs out inside the flea as the flea matures into a bloodsucking adult, but the tapeworm can reach adulthood itself only if the flea winds up accidentally being eaten by a dog or cat—or, more rarely, a human. Once inside its mammal host, the tapeworm larva gets into the animal’s gut, where it matures into an adult and starts producing eggs of its own. This starts the cycle all over again. 

Some parasites have impressive tricks for making it into the right host. Toxoplasma gondii uses mind control. This parasite can’t become an adult that lays eggs unless it’s inside a cat. But how does it get inside the cat? By infecting a temporary host first. Mice and rats sometimes eat food or drink water contaminated with a cat’s pooped-out T. gondii eggs. The eggs make their rodent host lose its fear of feline foes. This makes it easier for a cat to catch and eat the rodent. Then the cycle starts again. Pretty twisted, right? 

Parasites are doing predators a huge favor by messing with the behavior of their prey. It makes the prey an easy meal, even if a parasitic infection is for dessert. Scientists think that without parasites, it would be harder for lots of species to find food. Also, without them, we humans might suddenly be overrun by many of the creatures we find annoying. Imagine more rats and cockroaches that infest our homes and more pests that eat our crops. 

Many people don’t know about the important roles that parasites play because they write these organisms off as being gross and don’t want to learn more about them. Parasites are rarely covered in school, and few scientists study them—unless it’s to find ways to kill them. This lack of understanding is a problem because some parasite populations seem to be declining, which could signal larger environmental issues, but very few people are picking up on those signs. 

Wood and her colleagues discovered worrying changes to parasite populations in the Pacific Northwest’s Puget Sound over time through a very creative experiment. They borrowed jars of old, preserved fish from a museum and dissected the fish to see what parasites were found inside. This allowed them to figure out how parasite numbers changed over more than 100 years, from 1880 to 2019. They found that some parasites were disappearing and that those with the most complex life cycles, that involve infecting multiple hosts, were the ones that declined the most. 

The scientists used math to figure out that climate change seems to be causing these declines. This makes sense because as the planet is getting hotter, animals’ habitats are changing, their food is disappearing, and their health is deteriorating. For a parasite that relies on multiple species to stay alive, “it’s a really complicated dance, and you need everyone doing everything perfectly for it to work,” said Wood. 

Wood is guessing that similar patterns would emerge in other places outside her research area. Climate change “could be very, very bad for parasites,” she said.  

This means that some parasites might disappear before we even recognize the important roles that they play—and the truly astounding things that they have evolved to do. Making those discoveries and sharing them with others is Wood’s favorite part of her job. Once you peek into the secret world of parasites, she said, you’ll never look at life the same way again.